This invention relates to a method of producing beta-dicalcium silicate and more particularly to a method for producing fine particles which are reactive and suitable for use as a sorbent for desulfurization.
Beta-dicalcium silicate has been found to possess particular utility as sorbents because of their high resistance to attrition, high reactivity to SO.sub.2, high regeneration rate which permits lower temperature processing, and high reactivity of the regenerated sorbent. This high sulfation reactivity has rendered beta-dicalcium silicate particularly useful in industry in the desulfurization of high temperature combustion and fuel gases.
However, in the methods heretofore utilized for the preparation of beta-dicalcium silicate, which includes evaporative decomposition of solutions of calcium nitrate and colloidal silica at 1050.degree. C., or repetitive firing of a mixture of fine powders of calcium carbonate and silica at 1080.degree. C., the temperature is high and, therefore, the reactivity of the recovered beta-dicalcium silicate is rather low.
The preparation of calcium meta-silicates, calcium mesodisilicates and calcium silicate hydrates is generally known in the prior art.
For example, U.S. Pat. No. 3,264,130 to Mays discloses the process of reacting aqueous slurries of divalent basic metal oxides or hydroxides which include calcium oxide or hydroxide, with reactive silica in the molar ratio of 1 mole metal oxide per 4 moles silica, at elevated temperatures of about 100.degree. C. for a time sufficient to complete the reaction and recovering finely divided powdered calcium silicate pigment wherein 1 mole CaO combined with 1 mole SiO.sub.2 (CaO.SiO.sub.2) or with 2 moles SiO.sub.2 (CaO.2SiO.sub.2) as specifically described in Examples XIV and XV.
U.S. Pat. No. 3,988,419 to Mori discloses a process of producing calcium silicate insulation which comprises reacting an aqueous slurry of amorphous silica with lime at elevated temperatures of about 90.degree. C. to form a gel, kneading said gel under saturated steam pressure to convert to a viscoelastic material and subsequently treating said material with superheated steam to form xonotlite crystals of calcium silicate, which has the formula 5CaO.5SiO.sub.2.H.sub.2 O (Hackh's Chem. Dictionary, Third Edition, page 916).
U.S. Patent No. 3,131,024 to Leineweber discloses a process of preparing dicalcium silicate alpha hydrate useful as a flatting agent for clear lacquers by means of the hydrothermal reaction, at a temperature above 180.degree. C. (i.e., 232.degree. C.), of a slurry of calcium hydroxide and silica in the presence of a small amount of sodium hydroxide or fluoride. This process is disclosed as being superior to the prior art process of hydrolyzing beta calcium silicate at a temperature of 140.degree. to 200.degree. C., in the production of good yields of the alpha hydrate.
The Ohnemuller U.S. Pat. Nos. 3,861,935 and 3,967,974 disclose the process of preparing synthetic crystalline beta-wollastonite, which is calcium meta silicate (CaSiO.sub.3), by reacting an aqueous mixture of lime hydrate and silicic acid or silica, at a temperature of 100.degree. to 500.degree. C., in the presence of mineralizers (soluble alkali and alkaline earth metal salts) to form calcium silicate hydrates, and then annealing the hydrates at a temperature of 800.degree. to 1150.degree. C. to convert them to pure crystalline beta-wollastonite, the calcium silicate hydrate formed as the intermediate product prior to annealing being tobermorite or xonotlite.
U.S. Pat. No. 3,552,915 to Rostoker discloses a process making porous bodies of fibrous beta-wollastonite, which is a naturally occurring fibrous form of calcium meta-silicate, by sintering preformed bodies of beta-wollastonite at a temperature of 1150.degree. C., which is near but below the temperature at which beta-wollastonite undergoes a phase transformation to the alpha-wollastonite.
Although the prior art discloses the preparation of calcium meta-silicates, beta-wollastonite, wollastonite hydrates and dicalcium silicate alpha-hydrate, by reacting an aqueous slurry of lime or calcium hydroxide and silica at elevated temperatures of about 90.degree. to 500.degree. C.; there is no disclosure of the production of reactive beta-dicalcium silicate by a low temperature solid state process of firing a mixture of calcium sulfate and silica together with a reducing agent such as calcium sulfide or a member selected from the group consisting of C, CO, CH.sub.4 and H.sub.2 which reduces calcium sulfate to calcium sulfide.
It has now been found that in the present process, temperature can be as low as 850.degree. C. and up to 1000.degree. C., which results in substantial energy savings and yields more reactive products. In addition, the calcium sulfate reactant is more stable than the calcium reactants heretofore utilized. The beta-dicalcium silicate produced by the present process will possess superior sulfation reactivity, higher sorption reactivities and be highly regenerable from its sulfated compounds for reuse as a sorbent, as well as be more resistant to attrition.